(a) Field of the Invention:
The present invention relates to a finder optical system for single reflex cameras, and more particularly to a finder optical system suitable for electronic still cameras and video cameras of a single reflex type.
(b) Description of the Prior Art:
Most of the finder optical systems for conventional single reflex cameras using emulsion coated films as shown in FIG. 1, split a beam of light emanating from a photographing lens 1 time-dividedly using a beam splitting mirror 2 termed a quick return mirror the split beams of light are directed into a photographing system and an observing system, in which the light beam traveling to the photographing system is imaged on a film 6, while the light beam to the observing system is imaged on a screen mat and its image is established as an orthographic image by a pentagonal roof prism 4 for observance through an eyepiece 5.
In the electronic still cameras and video cameras, however, an image pickup element as small as 2/3-inch size (6.6 mm.times.8.8 mm) or 1/2-inch size (4.8 mm.times.6.4 mm) is employed lease are extremely small as compared with the size (24 mm.times.36 mm) of the emulsion coated film for a 35 mm camera, so that when the image is observed through a finder of the camera having an eyepiece with the same magnification as in the conventional 35 mm camera (with the emulsion coated film) the eyepiece cannot bring about the size of the image sufficient for observation. Therefore, in the electronic still cameras and video cameras, it is necessary to reduce the focal length and increase the magnification of the eyepiece.
However, in the case where the optical system depicted in FIG. 1 is regarded as a basic arrangement and an electronic image pickup device such as an image pickup tube or a solid-state image sensor is used instead of the film, various problems have been encountered such as, as shown in FIG. 2, the eyepiece 5 cannot be disposed directly behind the pentagonal roof prism 4 due to space occupied by an electronic image pickup device 8, an optical filter 7, and an electrical circuit 9 which are disposed in the front and rear of the image pickup device 8. Therefore, the optical path length between the screen mat 3 and the eyepiece 5 must be unnecessarily extended and this makes it difficult to reduce the focal length of the eyepiece 5 for increasing of the magnification.
In the past, with the intention of solving these problems, Japanese Patent Preliminary Publication No. Sho 61-29816 has been proposed, in which the magnification of the eyepiece is raised to prevent the optical path length from the focal plane to the eyepiece from being extended when the lens length of the eyepiece is increased. Further, Japanese Utility Model Preliminary Publication No. Sho 56-40531 has been proposed to provide a single reflex type camera which has a small image pickup surface on the film disk and renders a greater magnification of the eyepiece.
The above-mentioned Sho 61-29816, however, has raised difficulties because the eyepiece increases in size and the entire optical system becomes bulky due to the complicated lens configuration.
In addition, the above-mentioned Sho 56-40531, which makes use of a roof prism as a prism for observing the orthographic image, has met with a problem when the magnification of the eyepiece is increased, wherein the roof prism needs to be ground with high accuracy so that resolution is not diminished. This results in a high cost. Further, since the eyepiece needs to be placed to the rear of the film disk due to the mechanical construction of the system, the use of the roof prism (whose distance between the entrance position and the exit position of the light beam is long) renders the optical system larger and does not meet the intention of reducing the thickness of a camera body.